The present invention relates to a semiconductor device in which a drive circuit for driving a thermal print head and a logic control circuit for controlling the thermal head are each constituted by a P-channel MOS, an N-channel MOS, or a complementary MOS, or by a plurality of such devices.
A thermal print head 1 consists, as shown in FIGS. 1a and 1b, of a heating resistor 3, a drive circuit 4, and a logic control circuit 5 mounted on a ceramic substrate 2.
Since the heating resistor consumes a large current, a bipolar element has heretofore been used in the drive circuit 4 for driving the thermal print head, as shown in FIG. 3. As the temperature rises, the leakage current and the amplification factor of the bipolar element increase, so that the current driving the thermal head increases. Heat is generated by the increase in current, and hence a larger current flows. This phenomenon is usually known as thermal runaway. The heat generator in the thermal head generates heat. Therefore, when a bipolar element is mounted on the same substrate, conditions worsen and, if the substrate of the thermal head has a poor heat radiating efficiency, the bipolar element undergoes thermal breakdown. Thermal runaway and erroneous operations caused by heat present serious problems concerning the reliability of the thermal head.
Thermal print heads can be divided into the linear type and the serial type. In recent years, thermal heads of the linear type have been put into practice, and attention has been given to the generation of heat and consumption of large currents by bipolar elements. In the linear type of thermal head, 3 to 16 heat generator dots are provided per millimeter, and drivers are provided for each of them. For A-4 size, 1700 dots are required if 8 dots are provided per millimeter in the standard manner. Each dot must be supplied with a current of 20 mA to 80 mA to generate heat, which requires a power source with a large capacity. It therefore becomes necessary to reduce the current flowing into the logic circuit to as small as possible to minimize the load on the power source.
FIG. 3 shows the conventional setup of a thermal head employing a bipolar element. A bipolar logic control circuit 5a and a drive circuit 4a are supplied with a voltage of 4 to 6 volts from a power source 7, and a heat generator (not shown) is supplied with a high voltage of 15 to 30 volts. The logic control circuit 5a produces and output control signal for controlling the operation of the drive circuit 4a.